Ah. That doesn't surprise me in the least. And yes, that is a semilog plot. Probably the easiest thing to do is to fit a straight line to the semilog plot of your data. The idea is this: assume your function (on the original plot) is of the form

Here it is essential that both and

Now, plotting in a semilog plot is the same as taking the logarithm of both sides of this equation. The result is the following:

Your semilog plot has essentially plotted versus Therefore, you can find the slope and intercept of a straight-line-fit. The slope will be , and the intercept